1. Field of the Invention
The present invention relates to an image forming apparatus provided with an obliquely traveling sheet correcting device.
2. Description of the Related Art
In general, a position where an image is recorded with respect to the position of a sheet (hereinafter, referred to as "recording accuracy") is an important element of image quality in image forming apparatuses. Therefore, there are proposed various types of obliquely traveling sheet correcting means for enhancing the recording accuracy of conventional image forming apparatuses.
For example, there is widely employed an obliquely traveling sheet correcting device of a resist roller system which is arranged such that a pair of resist rollers are disposed in the direction perpendicular to a sheet conveying direction at a position nearest to an image forming unit upstream in the sheet conveying direction (hereinafter, simply referred to as "upstream") and the oblique travel of a sheet is corrected by causing the leading edge of the sheet to be abutted against the nip portion between the pair of resist rollers.
An example of a conventional image forming apparatus provided with an obliquely traveling sheet correcting device using the resist roller system will be described using FIG. 21. In FIG. 21, the leading edge of a sheet S fed from a sheet cassette 100 to a feed roller 101 is abutted against the nip portion between a pair of resist rollers 102 which have previously been stopped by a clutch or the like and the sheet S forms a curved loop as shown in FIG. 21 by the further rotation of the feed roller 101, whereby the oblique travel of the sheet is corrected by the stiffness of the sheet itself.
A resist sensor 103 is disposed upstream of the pair of resist rollers 102 in the vicinity thereof. After the leading edge of the sheet S is detected by the resist sensor 103, the pair of resist rollers 102 are rotated to cause a photoreceptor drum 105 disposed in a process cartridge 104 to convey the sheet S to a confronting image forming unit at a timing at which a predetermined period of time, during which the sheet S forms the loop and the oblique travel thereof is corrected as described above, has elapsed.
In the above prior art, however, the pair of resist rollers 102 and the feed roller 101 must be driven in rotation at a predetermined timing to cause the sheet S to form a predetermined loop between the pair of resist rollers 102 and the feed roller 101. As a result, a drive control system for them is complex and requires a high degree of control accuracy. Thus, a countermeasure such as the provision of a high performance clutch with a gear train is required.
Further, the pair of resist rollers 102 must be composed of a metal roller or the like having a low frictional resistance to reduce the frictional resistance made when the sheet S enters the nip portion between the pair of resist rollers 102. However, when the pair of resist rollers 102 are composed of metal, a pressurizing mechanism must be provided with the pair of resist rollers 102 to compensate for the insufficient conveying force of the sheet S. Since the pressurizing mechanism is relatively expensive and increases an occupying space in the device, there is the problem of the size of the device being increased as well as the cost thereof is made expensive.